1. Field of the Invention
The present invention generally relates to repairing and strengthening internally-reinforced concrete columns of structures and, more particularly, is concerned with a method of externally repairing and strengthing such columns in existing and new structures with a flexible strap of reinforcing material to increase the strength, stiffness and ductility thereof.
2. Definition of Terms
By way of definition, the term "concrete column", as used herein, is meant to refer to a structural element of a structure, where the structural element is hollow or solid and composed of internally-reinforced concrete primarily subjected to axial force, shear force, and bending moment. The term is synonymous with a bridge pier, pile, pillar, and post. The term also includes regions where beams or floor slabs frame into the column which are known in the art as joints or connections.
The term "structure" is meant to refer to any constructed facility wherein concrete is used, including, but not limited to, buildings, bridges, parking garages, factories, harbors and ports.
The term "repair" is meant to refer to the addition to or alteration of an existing structure for improved structural performance. The term "strengthening" is meant to refer to the addition to or alteration of an existing structure for the purpose of increasing the strength of the structure beyond its original value.
The term "strength" is meant to refer to ability to resist axial forces, shear forces, and bending forces. The term "stiffness" is meant to refer to the resistance to cracking and deformation. The term "ductility" is meant to refer to the ability of the structure to undergo permanent deformation prior to failure.
3. Description of the Problem
As known to those skilled in this art, a typical concrete column is internally reinforced with steel. Basically, the concrete column contains two types of steel reinforcement, as shown in FIGS. 1 through 6.
One type is a longitudinal steel reinforcement in the form of individual longitudinal rods or bars 10 which are spaced apart and placed internally along the length of the concrete column. The other type is a lateral steel reinforcement which is placed internally in substantially parallel relation to the exterior surface of the concrete column. The lateral steel reinforcement can be either in the form of individual rectangular hoops or ties 12, circular ties 14, or a continuous one-piece spiral rod 16.
The function of lateral reinforcement is to increase the shear strength of the concrete column and to provide confinement for the concrete 18 and lateral support for the longitudinal bars 10 to prevent them from buckling under large axial loads. Depending on the cross-sectional geometry of the concrete column, other shapes of lateral steel reinforcement can be used.
A variation of the above-described internally-reinforced concrete columns is one known to those skilled in the field as a "composite column", shown in FIGS. 7 and 8. The composite column is composed of a wide-flange steel beam 20, being H-shaped in cross-section, which is encased in the concrete column.
Concrete columns may require either repair or strengthening or both for various reasons. The repair or strengthening may call for the addition of external longitudinal or lateral steel reinforcement, or both of these. The reasons such repair or strengthening is needed include, but are not limited to, the following:
A. Seismic repair or strengthening
Many structures exist today which are not capable of resisting loads imposed on them during an earthquake. This is partly because when these structures were originally designed, little was known about how to design a structure to resist earthquake loads safely. As a result, many reinforced concrete columns in existing structures have insufficient longitudinal or lateral steel reinforcement or contain poorly-detailed steel reinforcement. Such concrete columns are unsafe in the event of an earthquake and therefore they need to be repaired or strengthened.
B. Gradual deterioration of structure
This deterioration could result from adverse environmental effects such as corrison of steel, salt spray, fire damage, hurricanes, tornadoes and the like. In such cases, the concrete column loses its design strength due to spalling of concrete and corrosion of reinforcement. Therefore, it is desirable to repair or strengthen these columns so that their strength is upgraded to at least that of the original capacity. This is a common problem with concrete columns in many aging structures.
C. Functional changes
In some structures, the introduction of heavier loads requires upgrading of load carrying capacity of concrete columns beyond their original design strength. For example, in order for older bridges to carry today's heavier trucks and traffic volumes, the strength of concrete columns must be increased beyond their original values.
D. Increased shear strength
Concrete columns in some existing structures may lack sufficient lateral steel reinforcement to withstand shear forces. In such cases, additional lateral reinforcement is needed to increase the shear strength of these concrete columns.
E. Increased ductility
In general, concrete columns with sufficient lateral steel reinforcement fail in a ductile manner, that is, they can resist large permanent deformations before they fail. Thus, repair and strengthening in the form of addition of lateral reinforcement may be desirable to increase the ductility of existing concrete columns.
F. Construction errors
Repair or strengthening may be required to correct some construction errors in a fairly new structure where, by mistake, some of the required reinforcement has been omitted or misplaced during the construction.
G. Increased factor of safety
Strengthening of some structures can be performed primarily for increasing the factor of safety against failure.
When repair or strengthening is required, it is necessary to employ the most cost effective technique. In selecting the appropriate repair or strengthening method, such factors as the original repair or strengthening cost and time required, future maintenance cost, expected life of the repaired or strengthened concrete column and the structure, availability of the repairing or strengthening materials, the ratio of the additional strength to cost, etc., should be considered.
For most concrete columns, the primary interest in repair or strengthening lies in providing additional confinement in the form of lateral reinforcement. Since it is not practical to add internal lateral reinforcement to an existing concrete column, some form of external lateral reinforcement is typically utilized.
4. Description of the Prior Art
Up to the present time, several methods known to those skilled in this art have been used to externally repair and strengthen internally-reinforced concrete columns in existing structures. These strengthening methods include, but are not limited to, the following:
1. Steel encasement
This strengthening method, also called steel jacketing, involves the building of a loosely-fitting steel case around an existing reinforced concrete column. The case is constructed of thin steel sheets and fully encloses the concrete column. The gap between the case and the column is then filled with pressurized grouting mortar.
2. Steel straps and angles
In this method of strengthening, steel angles are placed at corners of rectangular concrete columns along the full height of the column. Thin rectangular steel pieces are welded to the angles around the periphery of the column at specified elevations along the height of the column. This will create an encasing cage around the concrete column which will improve its structural response in the event of an earthquake.
3. Steel wire fabric
Welded wire fabrics in the form of orthogonal steel wires are placed around the periphery of the concrete column along the full height. A layer of fresh concrete is then cast on the wires around the column. This increases the cross-sectional area of the column and therefore its overall strength.
4. Closely-spaced external steel ties
This strengthening method is similar to strengthening with steel wire fabrics. Loosely-fitted steel ties are placed around the concrete column along its height. Concrete overlays are then cast on the ties to increase the size and therefore the strength of the concrete column beyond its original capacity.
5. High-strength steel wire
In this method, high-strength steel wires or strands are wrapped around the concrete column to enhance the ductility and strength of the column.
Although the above-described external strengthening methods help increase the strength and ductility of existing internally-reinforced concrete columns, they have several major shortcomings as follows:
A. Economy
These strengthening methods are all very labor-intensive and difficult to implement in the field. For example, they require field welding of steel, formwork for casting of additional concrete, and transportation of heavy equipment and concrete to the site.
B. Aesthetics
These strengthening methods will result in a significant alteration of the existing columns and may be objectionable and unsightly.
C. Applicability
Most of the strengthening methods described above are only suitable for application to prismatic members. For concrete columns whose cross-sectional size and shape vary along the height, these methods of strengthening could be hard or impossible to apply in the field.
D. Corrosion
The methods of strengthening by using steel encasement, steel straps and angles, and high-strength steel wire require further long-term protective measures to insure durability of steel casing against corrosion.
E. Size
The strengthening methods described above invariably result in an increase in the size of the concrete column. This will reduce the available floor space in buildings and adds to the self-weight of the structure.
F. Serviceability
The methods of strengthening described above enhance the response of the concrete column at the incipient of failure only. The serviceability of the concrete column would improve if the column could be laterally prestressed. Most of the above methods are not suitable for applying lateral prestress to the column.
G. Post-Earthquake inspection
Most of the methods described above fully cover the original concrete column. Consequently, after an earthquake, it will be impossible to inspect the extent of damage sustained by the column.
An alternative method, different from the prior art methods described above, which is asserted to provide concrete columns with sufficient lateral reinforcement in shear strength to be durable against earthquakes is disclosed in U.S. Pat. No. 4,786,341 to Kobatake et al. This patent discloses that, in accordance with the Kobatake et al method, a flexible reinforcing fiber strand is applied on the outer periphery of a concrete structural member, such as an existing concrete column, by spiraling winding the reinforcing fiber strand around the concrete structural member's outer periphery while impregnating the material of the reinforcing fiber strand with a resin. After the winding is completed, the patent discloses that the reinforcing fiber strand is pressed to expand it into a tape-like form having a certain large breath. By so doing, the patent discloses that the contact area of the reinforcing fiber strand increases, which relaxes the stress concentration, and delays the breakage of the reinforcing fiber strand.
The patent also discloses that the reinforcing fiber strand used in the Kobatake et al method can be a high strength strand in which about 6000 carbon fiber monofilaments are bundled and impregnated with a resin. The number of filaments may be adjusted. Alternatively, the reinforcing strand is disclosed as being formed of glass fiber or metal wire.
Also, in the Kobatake et al patent, it is disclosed that an insulating member can be interposed in an non-adhesive manner between the reinforcing fiber strand and the outer periphery of the concrete structural member. The patent mentions that the insulating material used should be one that will produce sliding between the concrete structural member and the insulating member or between the insulating member and the reinforcing fiber strand, or both.
In one example of the Kobatake et al method, the patent discloses that at the start of the winding operation the reinforcing fiber strand is first wound in a single winding turn around the outer periphery of the column in a direction orthogonal to the axis of the column to thereby form a hoop. After its starting end is bonded to the hoop by an adhesive, the reinforcing fiber strand is then spirally wound toward the upper end of the column. When it has reached the upper end of the column, the reinforcing fiber strand is again wound in a single winding turn in the direction orthogonal to the axis of the column to thereby form another hoop, and the terminal end of the reinforcing fiber strand is bonded to this latter hoop by an adhesive.
In this manner, the Kobatake et al patent discloses that, since it is possible to spirally wind the reinforcing fiber strand around the column by first bonding the starting end of the reinforcing fiber strand to the bottom hoop, it is thereby possible to impart a tensile force to the reinforcing fiber strand from the beginning and to provide the wound reinforcing fiber strand free from slackening or loosening, and hence in tight contact with the surface of the column. The Kobatake et al patent asserts that, since no tensile force is lost by bonding the terminal end of the reinforcing fiber strand to the top hoop, it is possible to realize the spiral winding of the reinforcing fiber strand free from the slackening or loosening. Also, since the reinforcing fiber strand is tightly wound around the column, the Kobatake et al patent asserts that the column receives the high binding force of the reinforcing fiber strand, whereby sufficient reinforcement is provided against earthquakes.
While the reinforcement method of the Kobatake et al patent may constitute a step in the right direction toward the goal of finding an adequate solution to the problem of how to repair and strengthen concrete columns, it appears to fall considerably short of achieving that goal. The winding of a reinforcing fiber strand around the concrete column would appear to be a time-consuming and tedious operation and produce concentrations of stress along the lines of contact of the fiber strand with the concrete column which would likely result in premature failure of the fiber strand and thereby of the external reinforcement provided by the strand.
Consequently, a need still urgently exists for a satisfactory approach for repairing and strengthening concrete columns.